1. Field of the Invention
This invention relates generally to ground proximity warning systems, and more particularly to a ground proximity warning system for aircraft of the type that provides warnings including warnings of inadequate terrain clearance wherein the criteria required to generate such a warning are modified when the aircraft is flying over undulating or mountainous terrain.
2. Description of the Prior Art
Ground proximity warning systems, including ground proximity warning system that provide warnings when the terrain clearance indicates a dangerous closure with terrain are known. Typical systems that provide a warning when the terrain clearance indicates a dangerous flight condition are disclosed in U.S. Pat. Nos. 3,936,796; 3,944,968 and 4,030,065. All of the above-mentioned patents are assigned to the same assignee as the assignee of the present invention and incorporated herein by reference.
While the warning system apparatus described in the above-mentioned references do provide satisfactory performance in most situations, they are essentially designed for the situation where an aircraft is flying over a relatively flat terrain. The warning system disclosed in the aforementioned patents provides warnings for inadequate terrain clearance up to 1000 feet of radio altitude. This is consistent with the FAA flight rules and in particular, Part 91, Subsection 91.119 for flight over relatively flat terrain. In that section, the minimum allowable altitude for an aircraft not landing or taking off is 1000 feet above the highest obstacle within a horizontal distance of five statute miles from the course to be flown. The respective flight rules concerning flight over undulating terrain or mountainous terrain dictate a minimum altitude of 2000 feet above the highest obstacle. The prior art warning systems, such as those disclosed, do not provide a warning to the pilot of an aircraft when the aircraft is flying above 1000 feet of radio altitude but over undulating or mountainous terrain and flying below the 2000 feet minimum altitude over undulating terrain. Since it is possible that aircraft will be flying over both undulating and flat terrain during the course of flight, the terrain clearance warning systems would not provide optimum results during all phases of flight. Thus there exists a need to provide a terrain clearance warning system that is modifiable when the aircraft is flying over undulating terrain to provide the pilot with a warning that the aircraft is below the prescribed altitude for undulating or mountainous terrain and to provide additional warning time to the pilot to permit appropriate action before impact.